Keratoconus is an eye disease that can cause distorted and blurry vision, even with glasses. This condition involves the thinning of the cornea (the window at the front of the eye), causing the surface to protrude forward in a cone shape, giving the disease its name ("keras" meaning cornea, and "konos" meaning cone). The condition is progressive, meaning it worsens over time, and with increased severity the cornea becomes more and more irregular.1 This can lead to astigmatism, scarring and severe vision loss.
Globally, keratoconus affects 1.3 people in 1000, although in Western countries like Australia these rates double.3 The reason why keratoconus develops varies from person to person, where genetic and environmental factors contribute to different degrees. Genetic factors include family history of the disease (especially first-degree relatives)4, Asian and Arabian ethnicity and congenital conditions like Down, Marfan and Ehlers-Danlos syndromes.2 Environmental factors include eye rubbing, allergies and hayfever, eczema, asthma and floppy eyelid syndrome.2 Often your optometrist will recommend certain anti-allergy eye drops to decrease symptoms of itchiness and reduce the urge to rub the eyes.5
Keratoconus presents during teenage years and progresses through to your 40s, although the age at which it worsens is variable.6 Most studies indicate that keratoconus affects men and women equally, and it is common for each eye to have a differing degree of severity.6,7
Keratoconus is a multifactorial disease, so the onset and progression can vary significantly between different people, and even between different eyes of the same person. Typically, night vision is affected first (haloes and streaks around headlights), which then develops into blurry and distorted vision.
Your optometrist can detect signs of keratoconus by analysing structural changes in the cornea. According to The Global Consensus on Keratoconus and Ectatic Diseases, for a true diagnosis of keratoconus there must be thinning at the back of the cornea that is not due to another cause, and an irregular corneal profile where some areas are abnormally thin.2 The only way this can be detected reliably is with corneal tomography.
Corneal tomography is a non-invasive way to measure the contours and shape of the cornea. It is the ‘gold standard’ in diagnosing and assessing keratoconus.4 Tomography refers to cross-sectional mapping, where the layers of the cornea are seen like layers of a cut cake. This gives a complete view of both the front and back surfaces of the cornea, both of which are needed to make a true diagnosis of keratoconus.2 This is unlike corneal topography, which only scans the front of the cornea, although it can still be useful in the management of keratoconus (for example, contact lens design).
Figure 1: Maps of the front, back and total thickness of the cornea (top), cross-sectional scan of the cornea (middle) and the effect this keratoconic corneal shape would have on vision (bottom).
Another non-invasive technique to map the front surface of the cornea. The images below are the topographical results of a relatively normal cornea (top) and keratoconic cornea (bottom). Note the regular rings on the normal scan compared to the distorted and compressed rings in the keratoconic. Areas of yellow and red represent areas that are more curved on the eye, and indicate the presence of a ‘cone’ in the keratoconic cornea. This has a significant impact on vision quality, especially when the distorted surface is within the pupil zone (the black circle).
Figure 2: The front surface map of a normal cornea (left) compared to a cornea with keratoconus (right).
The structure of the cornea can also be assessed using a slit lamp, which takes images of the front of the eye rather than scans. As well as changes in corneal shape and thickness, there are a number of other findings visible under microscopy. These include the presence of prominent corneal nerves, Fleischer’s iron ring and Vogt’s striae (corneal stress lines).8
Figure 3: The side profile of a keratoconic eye. Note that the shape is not round, but has a ‘cone’ jutting out toward the bottom of the cornea.
The anterior OCT is an invaluable tool that provides cross-sectional scans of the cornea, similarly to the corneal tomographer. The scans allow visualisation of the different layers of the cornea (as shown below) and also image the white part of the eye (sclera) close to the cornea. This is useful not only in diagnosing and monitoring keratoconus, but also for assessing the relationship between contact lenses and the eye. With this technology, we can achieve the most comfortable and safe contact lens fit to significantly improve vision.9 The example below of a custom scleral lens on the eye shows adequate space between the contact lens and the cornea, and the area at the edge of the lens where it lands smoothly on the sclera.
Keratoconus is very unpredictable and can progress rapidly over a short period of time, before stabilising for months or years, then progressing again.10 For this reason, regular eye tests are recommended.
Vision can be adequately improved in some keratoconics with glasses or soft contact lenses. However, with disease progression these become less and less effective. At this point, several options can be considered in consultation with your optometrist.
Cease eye rubbing: Rubbing your eyes is extremely detrimental to the keratoconic cornea and promotes progression, even in people with no other risk factors. Lubricating eye drops can help to stop this urge. Similarly, for patients with allergies and hayfever who are tempted to itch their eyes, anti-allergy eye drops including Cromo-Fresh can be used.
Bespoke contact lenses: This includes rigid gas permeable (RGP) and scleral lenses. All optometrists at Innovative Eye Care have a strong background in keratoconus management and research, using EyeSpace to design rigid lenses for our patients. It’s important for keratoconic patients to be assessed and fitted by an experienced optometrist. Poorly fitting contact lenses have been shown to promote the progression of the disease in some individuals and can cause permanent vision loss through scarring.11,12
Figure 5: Vision in a keratoconic eye without correction (left) compared to vision in the same eye with a scleral lens (right).
Corneal cross-linking: If keratoconus is diagnosed early enough, collagen cross-linking (CXL) is often recommended. For this procedure, riboflavin (Vitamin B2) is soaked into the cornea and activated by UV light. This promotes cross-linking of the collagen fibres in the cornea, leading to a strengthening of the tissue. Collagen cross-linking will not cure someone of keratoconus and it may not improve vision, but decreases the chance of further thinning and damage.13 If your cornea has become too thin (~400um) then CXL is too unsafe to be carried out, so early referral is important.14
Corneal graft: When keratoconus is no longer progressing, corneal grafts can be an option for those who still have poor vision with glasses or contact lenses. This can be due to corneal scarring as a direct result of keratoconus or because of corneal hydrops, a condition where the back surface of the cornea ruptures due to extreme thinning. A corneal graft may also be used when contact lenses are no longer tolerable to wear. However, grafts have a risk of rejection, require long courses of steroid drops, and often require spectacles or contact lenses after the surgery.
Intracorneal ring segments (ICRS): In certain cases, plastic rings can be inserted into the cornea to both artificially thicken and flatten the cornea. This is only an option for those with mild short-sightedness (under –5D) for whom contact lenses and glasses are unsatisfactory, and even then will need a consultation with an ophthalmologist to determine suitability.
Pellucid Marginal Corneal Degeneration (PMCD) is an uncommon condition causing thinning in a different part of the cornea (the bottom-most area). This condition is often confused with keratoconus as it causes similar corneal changes and requires complex contact lens designs, typically in the form of scleral lenses, to give the best quality of vision. Patients will often present with blurred vision due to the irregularity of their cornea. Often against-the-rule astigmatism is present and contributes to the blur. In PMCD, the stroma (middle layer) of the cornea is abnormal and thin but the other parts of the cornea are unaffected. This degeneration can progress slowly over time, but will only very rarely progress to the point of perforating the cornea.
PMCD is best managed with large diameter scleral contact lenses that will vault over the affected area of cornea and still provide a regular front surface for vision. In severe cases a corneal graft may be needed to restore functional vision. Collagen cross-linking is a surgical procedure used in some progressive cases to prevent further worsening of the condition.